Siege Engine.com: How to Make Slings and Pouches
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Our Trebuchets and Onagers use throwing pouches. The advantage to a pouch on a sling is that it adds speed to the projectile beyond what is possible in the throwing beam.

We have had many pouches, and several have proven useless after a certain scale. The type of pouch you use for your own machine depends on what size of machine you have, and what skills you have available.


Trebuchet Sling

A classic fabric sling is just that, a piece of fabric with rope tied to either end. In this case, Treb Jr's sling is made of denim. This is the second sling made for this machine since the holes in the denim tend to pull out after several shots.


This pouch was made by Amy for Juggernaut, and is just the right size for 20oz soda bottles. 2 liter, and gallons do fit, but we usually use a large pouch for them. The fabric is canvas from an Army tent that was decommissioned. It was folded over so there are six layers of fabric where the grommets are set.

This style pouch works well for our traction trebuchet, but this is the third one we've had to make. The previous two both had the grommets pull out from throwing objects that are too heavy. This is a classic problem with pouches of this design. The dilemma is that the material was not designed to be used in the same fashion as the rope it is tied to.


To overcome the problems we had with our pouches tearing for large projectiles, Roger came up with this one. We bought two basket ball nets, cut them open, and linked them together with some clothes line. We use the quick links to hold the ends together. We also use those quick links to easily swap this pouch out with the previously described pouch.

This sling was also used with our larger counter weight trebuchet Juggernaut 2.


Trebuchet Sling Length


It is often important to get the length of your sling lines correct for trebuchets. The length you need is a combination of counterweight, arm length, and projectile size.

The rule we used was, when the counterweight and projectile weight were well balanced, the length of the sling line (L-3 in this image from Seuss Trebuchet) is the same as the distance from the fulcrum to the release hook.

In addition, we always tie our sling line a short distance from the hook itself. We've done this with success on all our trebuchets, and on our Onager as well. This forces the projectile to roll slightly in the sling as it spins around the end of the arm. The result is a nice top spin which (theoretically) can improve distance the same way a golf ball can float longer with top spin.

You should never make the sling longer than this, in that if the projectile starts too close to the fulcrum, it will never get any energy imparted to it. Shorter slings can be used if your projectile is too heavy. The shorter lines make it easier for the projectile to accelerate faster than the arm. When a projectile has a flat trajectory, often the sling will be too short, or the projectile just isn't heavy enough.


Onager Sling


Onager slings have the same issues as those for the trebuchet. After pulling several grommets on our trebuchet, I opted to have
Amy sew rope into the edges of our familiar trebuchet sling design. This worked well in that we didn't pull out any grommets.


Unfortunately, the high acceleration of the onager completely overpowers the fabric where it is over the rope. I thought the stitches might pull out, but it just ripped right through the canvas.


When Eric saw Amy knitting Afghans with size 35 knitting needles (about 3/4 inch in diameter) he insisted that she try knitting a pouch out of some nylon clothes line. This pouch can take everything the onager can dish out. I doubt I'll need to have another pouch made.


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Twas' brillig, and the slithy toves did gyre and gimble in the wabe...
Last Modified: 11/05/12